Grinding mill control



Dec. 19, 195 F. c. BoND GRINDING MILL coNTRoL Filed p1-11 1e. 1945 @AMPA/N6 .DIS C .l I I I I i I Patented Dec. 1950 oNlro s'rars PAENT oFFicE arnese l namento anni. contraen lllred C. Bond, West Allis, Wis., assigner to Allis- Chalmers Mauniiaoturing Company, Milwaukee, Wis., a corporation ai .Delaware application April. 1i, i945, Serial No. 588,463

(Cl. 2il=2l) ing mill operation depended primarily on the amount of material in the mill and the character of the grinding means, and were subject to certain inaccuracies which are inherent therein.

The present invention represents a new departure in grinding mill control, in which the rate of feeding materials into the mill for grind lng is controlled by a force related not to the quantity of material in the mili, but directly to a physical characteristic of the product produced by the mill which it ls desired to control.

An object of the invention is to provide method of automatically grinding material so as t) maintain a product of substantially constant desired specific surface.

Another object of this invention is to provide apparatus for automatically grinding materials comprising a device for governing the operation of the apparatus in response to changes in the specic surface of the product.

The invention having the above and still rurther objects and advantages, which may appear hereinbelow, may best be carried into elect as described herein with reference to the accomn panying drawing, in which:

Fig. l is a schematic diagram of a system of apparatus for carrying out the invention and showing the invention applied to the control of a conventional ball mill in closed circuit with a conventional rake classifier;

Fig. 2 is a schematic diagram showing to en larged scale and in cross section a suitable sam pling and measuring device such as is shown at the right-hand. side of Fig. 1;

Fig. 3 represents a modification of the material feeding part of the system shown in Fig. i;

Fig. 4 is a view of the discharge end of a grinding mill with modied'sampling and measuring apparatus suitable to control its operation according to the invention.

lIn the production of certain finely divided solids, the average neness or' the product or the specific surface ci the product 'is desired to be kept as close as possible to a flxed standard. This is true for example of the production of Portland cement where it is more important that the specific surface of the product have a certain value than that the particles be all of a certain minimum size. This is also true of the gli production of certain nely divided catalysts, ores, and many other products both mineral and nonmineral in character. It is of course desirable to make the desired product as economically as possible, and for this reason grinding, which consumes time, power and money, should be kept at a minimum necessary to produce a satisfactory product.

In the accompanying drawing the present invention has been illustrated as carried out in connection with grinding solids in a mill of the so-called ball mill or "tube mill type. It is obviously applicable, however, to other types of grinding mills. A conventional continuously,

rotating grinding drum I is shown in Fig. 1, arranged to rotate on a substantially horizontal axis. The mill I is provided with a feed hopper 2 receiving a stream of material to be ground and guiding it into the mill I. A rotary scoop feeder 3 may also be provided to return partially ground particles and grinding media to the mill from the coarse material outlet of a classifier I of conventional type. The discharge spout B of the mill i is shown in position to discharge materials into the classier 4 which may be for example of the rake type, commonly used 1n circuit with ball and tube mills. Classii'ler I is provided, for the purposes of this invention, with a. sampling disk l. Disk 1 may have rockable cups 8 which dip into the pulp or uid suspension in the classifier near the overflow weir 9. Cups 8 are normally kept upright as by pendant weights il and are tripped by any suitable known tripping mechanism (shown schematically) as they pass the hopper I2 of a sample mixer I3, so as to discharge their contents into the hopper I2. The disk E may be driven at a. constant speed by any suitable means such as constant speed motor 54.

The above described structure is largely conventional, and since its operation is well known to persons skilled in the art, a detailed explanation will not be necessary except as to the action of the Ferris wheel sampling disk 1 by means of which, it will be understood, a continuous series of samples of the lines or pulp in the classifier near the overow will be discharged into hopper l2 at a constant rate, each sample containing a xed volume of the uid mixture or pulp from classifier 4.

A variable rate feeder or conveyor is shown at lli. This may be of the well known type in which a belt conveyor I'I is supported in part by a weighing balance I8 which is arranged to regulate a gate I9 controlling flow of material to be ground from feed bin 2I to the belt conveyor l1. Material on belt conveyor I'I is conveyed to hopper 2. For the purposes of this invention the conveyor I'I may be driven at a variable speed by a driving motor 22. A valved pipe 23 may be arranged to supply a transparent liquid grinding 3 medium. such as water, to the hopper 2 to enter the mill I with the material to be ground.

In Fig. 2 the apparatus shown at the right side of Fig. 1 appears more in detail. The mixer I3 is shown as provided with an agitating paddle 24 rotated by motor I4. An inlet pipe 26 admits materials received by mixer hopper I2. A diluent inlet 21 is arranged to admit a predetermined iiow of water or other diluent into mixer hopper I2. A circulating lift 28 operated as by a pump 29 continually circulates the fluid suspension formed in the mixer I3, taking mixture from the bottom and returning it to the top of the mixer. Overflow pipe 3|, which may be provided with a valve 32, leads the mixture from the mixer I3 to the top of a turbidimeter chamber 33 having opposed windows 34, and a lter rack 35 for a purpose described hereinafter.

Material from sample mixer I3 iiows at a constant rate through chamber 33, and is carried off from the bottom through pipe 36. A valved bypass pipe 31 may be provided to regulate the ow rate through the turbidimeter chamber.

As air or other gas bubbles may in some cases interfere with accurate control, it is contemplated that a deaerator I of known suitable type, may be inserted in the sample circulating system, as for example between the hopper I2 and mixer I 3. Since suitable deaerators are known to the art. it will be unnecessary to illustrate or describe, in detail, their structure or operation. It will be suiilcient, to those skilled in the art, to state that it may be of the type in which the liquid mixture is conducted through a chamber in which a partial vacuum is maintained.

A constant light source 38 of any known suitable type, which may be adjustable as for example by a rheostat 38. is arranged to direct a beam of light through the windows 34 of turbidimeter chamber 33 to a light sensitive element or photoelectric cell 39 of any suitable known type, such as one in which the electrical resistance varies inversely to the amount of the light impinging upon it. A galvanometer 4I may be supplied to indicate the light transmitted through the turbidimeter and may be calibrated to indicate the turbidity in the chamber 33. The electric current ln the photoelectric cell circuit may be impressed on a suitable electric motor speed control 40 of known suitable type, the output current of which may be arranged to control the speed of motor 22 which drives the conveyer belt I1.

'I'he light sensitive apparatus schematically shown is not necessarily constantly excited. It will be evident to persons skilled in the art, that the light sensitive cell 39 may only be energized intermittently and that when the word continually is used herein, and in the claims, it should be understood to have its broad meaning, which includes: in regular or repeated succession.

The control of the speed of motor 22 by the speed control element 40 may be continuous, with the motor speed (feed rate) constantly proportional to the output of the light sensitive cell 39; or the speed control 40 may be of any known type which will intermittently vary the motor speed by control impulses proportional to the deviation of the output of cell 39 from a predetermined normal, the feed rate being maintained in such relation to the turbidity of the mixture in chamber 33 as to correct for deviation vfrom a predetermined normal turbidity.

It will be understood that the variable speed motor 22 may, alternatively, be replaced by a constant speed motor operated intermittently according to the controlling current from the light sensitive device. Suitable electrical control circuits for speed control 4I1 to control the motor 22 to give a feed rate proportional to the controlling current from cell 39 are known to persons skilled in the art of motor control and need not be described at length in this specification.

Fig. 3 shows a modication of the feeding device ot Fig. 1, in which a known mechanism for controlling a supply of water or other liquid me- ,dium from pipe 23 is used, so that the liquid will be supplied in constant proportion to the rate of feed of solid materials, as by use of a metering pump 43 discharging into hopper 2. Pump 43 may be driven by a crank from the feeder drive motor 22, its rate, thus, being kept proportional to the rate of feed of solids.

Fig. 4 shows a modification of the sampling device of Fig. 1 suitable for application to a wet, or dry, open-circuit, or closed-circuit, mill. In this modification, sampler disk 1 is replaced by a similar sampling device 44 of suitable known type, such as a sampling cup 44, suitably secured to the mill, or otherwise supported and driven, arranged to take uniform samples of material discharged from mill spout 6. These samples are led to a surge hopper 45 of a small suitable sample feeder 46 which delivers the samples in an even, steady stream to mixer hopper I2. In the case of closed-circuit grinding, according to this modication, the sample of the product is taken between the mill spout and classifier, instead of from the fines in the classifier.

'I'he circuit for dry, closed-circuit grinding may be similar to Fig. 1, with fluid inlet 23 omitted and a dry classifier, such as an air separator of suitable known type (not shown), substituted for the rake classifier 4. The sampler disk 'I may be replaced by any suitable sampler (not shown) taking a uniform sample from the lines of the dry classifier or air separator.

Where intermittent operation of motor 22 is contemplated, a constant speed motor is used and the feeder gate is so adjusted as to give a feed rate somewhat greater than that which would normally give the desired grinding action. The mill will illl until the product size begins to grow coarser than the size desired, whereupon the light sensitive device will act to stop the motor. The product discharged will then gradually grow finer, due to reduced flow through the mill, until the light sensitive device indicates that the product size is again satisfactory, whereupon the motor will start and over-feed somewhat until the product again grows too coarse. When this system is used, it will be desirable to cut oil. the liquid medium feed when the feeder cuts oil, or meter the liquid feed as shown in Fig. 2. If it is necessary to cut oi! liquid supply, a solenoid operated valve 41 of known suitable type in pipe 23, may be controlled by the motor speed control 40, to open when the motor 22 is running, and close when the motor is stopped.

Materials are automatically ground, substantially to a predetermined specic surface, according to the invention, by feeding the materials at a controlled rate into a mill I of a type which operates at a constant predetermined speed and is capable of doing a certain amount of grinding work on the materials as they pass continuously through it. In the apparatus shown in Fig. 1. the materials are ground wet, as in water or some other transparent liquid medium. The suspension of ground and partly ground materials anzu-,ese

or pulp discharged through the mill spout 8 enters a classifier 4 where it is dispersed in a larger amount of liquid. Coarser particles sink more rapidly than ne and are raked out and returned to the mill l for further grinding therein. A constant series of samples of the pulp, or fines suspended in liquid, is taken from the classifier, near the overflow, by the sampler disk l, and delivered to mixer i3. In the mixer the pulp sample is dispersed in a constant proportional amount of diluent liquid, perhaps at a ratio of about one thousand parts of water to one part of fine solids for fine grinding and somewhat less for coarser grinding.

In the mixer it the solids and diluent liquids are thoroughly agitated by stirring paddle 2l and circulated constantly by the pump 29 so that the overflow from the mixer is of substantially uniform density. The irregularity of intermittent feed by the sampling disk 'l is thoroughly smoothed out, and the mixture made fairly representative of the classier overflow product.

The light beam from source 38 is partly absorbed and reduced in density by the particles suspended in the fluid in turbidimeter 33 to an extent dependent on the specific surface of the solids in suspension in the path of the beam through chamber 33. The light sensitive device 39 will indicate the turbidity of the fluid, which is related to the specific surface of the ground solids in suspension in the transparent liquid. The current or E. M. F. response of the light sensitive device will cause the motor speed control 40 to exert a controlling force which will control the speed of motor 22 of the feeder in a manner to regulate the feed rate substantiallyin accordance with the specific surface of the mill product.

It will be apparent from the foregoing description that when the ground material being discharged from the mill or classifier gets too coarse (specific surface reduced), the amount of work the mill is doing on the quantity of material being fed thereto is insufficient to reduce all the feed to the desired product size, and the amount of feed should be reduced. Assuming a constant ratio of solids to liquids in the samples and sample mixer and turbidimeter, the controlling current or E. M. F. produced by the light sensitive device will be proportional to the specific surface of the product of the mill, and, if too low a value of specific surface is indicated, the rate of speed of the motor 22 will be reduced, or its operation interrupted, until the product size returns to normal.

Now in the device as shown if Fig. 3 or 4 is employed, the above action takes place as described, the weight and ratio of sample solids to liquid in the turbidimeter being substantially constant. With this arrangement, a change in the size or character of the feed, causing an increase in the amount of work to be done to reduce a given quantity to the desired product size, will result in a lowering of the specific surface, which'will lower the feed rate and tend to increase the specific surface of the product. llf the feed retains the changed character for a long period of time, the decrease of feed, which occurs when the specific surface begins to increase, will cause the light responsive device to tend to bring the feed rate back to the original rate, which will again result in too coarse a product. The action will become stabilized at a'slightly coarser product and lowered feed rate, so long as the feed character remains abnormal, but the mill will be maintained at a high rate of etliciency and the product will be maintained at near the desired size.

If the mill becomes underloaded, the opposite action will take place, that is, the product will begin to become too line, or have too high a specific surface, indicating uneconomical or wasteful excessive grinding. In such a case, the turbidity being increased, the light sensitive device will act to increase the feed rate and the device will become stabilized at a slightly increased feed rate and only slightly overground product, the mill efllciency being maintained at a high figure.

A closer control of product size may be maintained in a wet, closed-circuit system, as .shown in Fig. 1, by maintaining the flow of liquid grinding medium to the mill, classifier and mixer constant while the rate of solid feed only is varied by the light sensitive device. In this case a decrease of specific surface in the mill product will cause, as described above, a lowered feed rate, but the dilution of the mill charge, classifier pulp and sampling mixer suspension will then be increased, that is, there will be a smaller weight of solids in a given volume of mixture in the tur bidimeter after a reduction of solid feed rate. 'When this state of operation persists, the turbidity, instead of increasing in proportion to the decrease in feed rate, will tend to increase at a lower rate, on account of the decreased quantity of solids in the turbidimeter chamber. Thus, when the feed becomes more diflicult to grind, the turbidity will be lowered, lowering the feed rate and the ratio of solids to liquids in the suspension.

As the mill begins to grind finer again, at the lowered feed rate, the resulting increase in turbidity at greater dilution will not return the feed to its original rate, so long as the character of feed remains more difficult to grind, because the same specific surface of product will produce a lower turbidity of a more diluted mixture in the turbidimeter. The feed will be stabilized at a lower rate, so long as the product remains more difllcult to grind to the specific surface desired. This is true because, so long as it remains more diicult for the mill to grind to a required size, if the turbidity tends to increase at the increased dilution, due to too low a feed rate, every increase in feed rate by the turbidimeter will tend to lower the dilution in the turbidimeter due to the constant water rate, and when the feed rate becomes high enough to return the product size to the predetermined specific surface, the turbidity will still be at the increased ligure on account of the decreased dilution at the new feed rate. Accordl ingly, the arrangement shown in Fig. 1, when properly adjusted to operate with fixed predetermined liquid supply, will automatically regulate the mill to give substantially the desired product size, within practical limits, in spite of changes in initial size or character of solid feed.

The arrangement of Fig. 1, withconstant liquid supply, should be used where substantial fluctuation in material feed characteristics is likelyl to occur, and the arrangement of Fig. 3, with liquid supply proportional to solid feed, may be used where feed characteristics are likely to remain fairly constant.

To initially adjust the light sensitive control, the mill is set up to operate with a certain definite feed rate and the feed rate is then manually adjusted to stabilize the mill to produce approximately the desired specific surface in the product. The controlling current output of the controller 40 is then adjusted to a value suitable to run the mill feeder at the proper speed to obtain the desired feed rate and product. The feeder speed may be adjusted, in an obvious manner, by changing the intensity of the light beam produced by source 38, or changing the excitation of the light sensitive cell 89, or by regulating the speed controll.

The light source I8 may be checked. or adjusted. as by means of rheostat 38', to the proper intensity, by the use of a light filter placed in the light filter rack 35, with the turbidimeter bypassed and/or filled only with transparent liquid, without changing the adjustment of the light sensitive controlling circuit. The galvanometer 4l should have a certain predetermined reading for any given setting with the checking filter in place.

It will be understood that any known suitable light responsive turbidity indicator and control circuit, may be utilized in an embodiment of the present invention within the scope of the claims hereto appended. It will also be apparent that the electrical control 40 of feed motor speed may involve known suitable automatic time delay devices (not shown) to prevent hunting and excessive fluctuation due to momentary conditions. The simple constant light source 38 and photoelectric cell 39 are only illustrative of the elementary principle of electrical turbidity measurement.

While specific apparatus has been illustrated and described herein, it will be understood that the invention is intended to include adaptation to other types of grinding machines (not shown) and such other modifications and equivalents as may occur to persons skilled in the art within the scope of the appended claims.

It is claimed and desired to secure by Letters Patent:

1. The method of grinding materials in a continuous grinding mill comprising the steps of feeding material to be ground into said mill at a variable rate, taking a substantially continuous sample of the product discharged from the mill, creating an agitated suspension of said sample in a predetermined proportional quantity of a transparent uid medium, measuring a light transmission characteristic of said suspension, producing an electric current varying in value in response to changes in said light transmission characteristic so determined, and applying said electric current to the control of the rate of feed of material to said mill.

2. The method of operating a continuous grinding mill comprising the steps of feeding materials to be ground into said mill at a variable rate, classifying the mill discharge and continuously returning the coarser fraction to the feed entering the mill, taking a substantially continuous sample of the ner fraction from the classifier, creating a suspension of said sample in a predetermined quantity of a transparent fluid medium, measuring a light transmission characteristic of said fluid suspension, producing an electric current varying in value in response to changes in the light transmission characteristic so determined and applying said current to the control of the rate of feed of materials to said mill.

3. The method of grinding material in a continuous grinding mill comprising the steps of feeding material to be ground into said mill at a variable rate, feeding a. transparent liquid medium into said mill with said material at a predetermined constant rate, measuring a light transmission characteristic of the mixture of comminuted materials and transparent liquid discharged from said mill, producing an electric current varying in value in response to changes in said light transmission characteristic so determined, and applying said electric current to the control of the rate of feed of said material to said mill.

4. In a system for grinding materials, a grinding mill continuously operating at optimum speed, means for feeding materials to be ground into said mill at a variable rate, means for discharging ground materials from said mill, means for taking a substantially continuous sample of predetermined quantity of the material discharged from said mill, means for producing a suspension of said sample in a predetermined quantity of a transparent uid medium, a light source for producing a light beam of substantially constant value and causing said beam to pass through a predetermined stream of said fluid suspension, light sensitive ymeans for producing an electric current varying in response to changes in a light transmission characteristic said stream of fluid suspension, and means for controlling said material feeding means in response to changes in the value of said electric current to correct for variations in the physical characteristics of the mill 'product as indicated by the light transmission characteristic of said suspension.

5. A system for controlling the operation of a grinding mill operating continuously in closed circuit with a classifier discharging the ground product and returning insufficiently ground material to the mill for regrinding, comprising means for feeding material to be ground into said mill at a variable rate and means for feeding a transparent fluid grinding medium into said mill at a substantially constant rate, means for discharging a mixture of ground material and grinding medium from said mill into said classifier, means for continually taking a sample of substantially constant quantity from the mixture of finely ground product and transparent fluid medium discharged by said classifier and mixing the same in substantially constant proportions with additional transparent fluid medium, means for continually measuring the turbidity of the mixture so produced and producing an electric current varying in value in response to changes in the turbidity, and current responsive means associated with said feeding means to control the rate of feed of unground material to said mill in a manner to maintain the turbidity of said suspension substantially at a predetermined value.

FRED C. BOND.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,294,759 Blomfield Feb. 18, 1919 1,294,864 Blomfleld Feb. 18, 1919 1,619,807 Blomfield Mar. 8, 1927 1,970,613 McDill Aug. 21, 1934 FOREIGN PATENTS Number Country Date 552,398 Great Britain Apr. 6, 1943 

